Growth of Ag thin films on complex surfaces of quasicrystals and approximant phases

The nucleation and growth of Ag thin films deposited on structurally complex alloy substrates is studied by scanning tunneling microscopy and low-energy electron diffraction. The substrates are high-symmetry surfaces of either icosahedral or decagonal quasicrystals or of related periodic crystals called approximants. At low coverage, completely different film morphologies are observed depending on the surface free energy of the substrate (relative to Ag). At high coverage, the formation of hexagonal nanocrystals is observed in all cases. These have fcc symmetry and pyramid-like multilayer stacking along the 〈1 1 1〉 direction. The constituent hexagonal islands have one, two or five different orientations depending on the rotational symmetry of the substrate on which the growth occurred. The films can be artificially smoothened by O2 induced coarsening, leading to a patchwork of Ag nanocrystals with a perfect texture, which are separated by grain boundaries.